This invention relates to a polymer electrolyte and more particularly a polymer electrolyte suited for use in lithium batteries.
In recent years, lithium ion batteries have been put to practical use and enjoyed wide use in various electronic equipment. In particular, lithium secondary batteries using metallic lithium or a lithium alloy with other metals as a negative electrode are expected as promising secondary batteries having high energy density. However, the state-of-the-art lithium secondary batteries involve several problems that have hindered them from being put on the market. The biggest problem of our concerns waiting for solutions is how to prevent generation and growth of lithium dendrites during charging processes. The problem of dendrite formation also occurs in a negative electrode comprising a lithium-intercalated carbon material under the condition of a high rate.
Allowed to keep growing, lithium dendrites will reach the positive electrode of a battery to cause an internal short-circuit. In case an internal short-circuit should take place, a large current instantaneously passes through the dendrites, resulting in generation of temperature increase and pressure increase, which may lead to take a fire. Therefore, various means have been tried for preventing such an internal short-circuit. To prevent an internal short-circuit would extend the battery performance life and maintain the high value of charge and discharge efficiency. In JP-A-60-167280, for example, a rechargable electrochemical device in which formation of lithium dendrites is suppressed by using an alloy of lithium and other metals has been disclosed.
Use of an ion-conducting inorganic solid electrolyte, polymer electrolyte or solid polymer electrolyte, etc. for suppressing growth of lithium dendrites has also been under study. For example, Oyama et al. have reported that a polyacrylonitrile (PAN) gel electrolyte, in a concentration of 5% by weight or more based on a nonaqueous solvent, protects lithium surfaces from forming dendrites (New Energy and Industrial Technology Growth organization (NEDO) ""96 Research Report (Mar., 1996))
It is expected for lithium batteries and capacitors which are to be developed to have not only an increased energy density but capability of rapidly working within limited charging and discharging times. In particular, growth of batteries which function sufficiently in low temperature (xe2x88x9220xc2x0 C.) has been sought for.
In general, the performance of batteries and capacitors is, in nature of their working principle, limited by the ionic mobility and the distance of ions to be transferred. In the case of a battery, since it is impossible to appreciably increase the ionic mobility in the electrolyte and in the electrode active material, an approach to be taken is to shorten the distance of ions to be moved and to use a material having a large reactive area. In the case of a capacitor, too, increased mobility of carrier ions leads to considerable reduction of the charging and discharging times. Therefore, in order to improve capacitor performance, it is necessary to shorten the distance between electrodes and to widen the electrode area as with the case of batteries. To materialize the above approach, it is essential to prepare a very thin and yet mechanically strong electrolyte film.
Further, a secondary battery using metallic lithium as a negative electrode has also been demanded. As stated above, however, a battery having a negative electrode of metallic lithium and a liquid electrolyte suffers from growth of lithium dendrites on the interface between the negative electrode and the liquid electrolyte on repetition of charge and discharge cycles, which gives rise to deterioration of battery performance and the safety problem.
Polymers, when applied as battery materials, have advantages of lightness, flexibility, and capability of thin film formation and are therefore promising for providing a next generation of batteries. A polymer electrolyte comprising a polymer and an organic solvent containing an electrolyte is particularly sought for. However, a polymer gel is disadvantageous in that, for one thing, a reaction current is concentrated at part of the negative electrode surface because lithium ions are transported via the solution phase in the polymer matrix as is observed with a type of solution electrolyte and, as a result, lithium deposits locally to induce growth of lithium dendrites. For another thing, a polymer gel has weaker mechanical strength than a solid polymer.
Further, conventional solid or gel polymer electrolytes fail to function sufficiently in low temperature. In addition, a gel polymer has poor liquid retentive properties in high temperature.
Accordingly, an object of the invention is to provide a novel polymer electrolyte having high ionic conductivity.
Another object of the invention is to provide a polymer electrolyte which suppresses growth of lithium dendrites.
Still another object of the invention is to provide a polymer electrolyte which provides a battery exhibiting excellent discharge characteristics in low to high temperatures.
The present invention provides in its first aspect a polymer electrolyte comprising a polymer gel holding a nonaqueous solvent containing an electrolyte, wherein the polymer gel comprises (I) a unit derived from at least one monomer having one copolymerizable vinyl group and (II) a unit derived from at least one compound selected from the group consisting of (II-a) a compound having two acryloyl groups and a (poly)oxyethylene group, (II-b) a compound having one acryloyl group and a (poly)oxyethylene group, and (II-c) a glycidyl ether compound.
The invention also provides in its second aspect a polymer electrolyte comprising a polymer gel holding a nonaqueous solvent containing an electrolyte, wherein the polymer gel comprises (A) a unit derived from at least one monomer having one copolymerizable vinyl group, (B) a unit derived from a compound having two acryloyl groups and a (poly)oxyethylene group, and (C) a unit derived from a plasticizing compound having a polymerizable group.
The term xe2x80x9c(poly)oxyethylene groupxe2x80x9d as used herein is intended to include both an oxyethylene group and a polyoxyethylene group.
The polymer electrolyte according to the invention suppresses growth of dendrites. In particular, the polymer electrolyte according to the second aspect exhibits excellent low-temperature characteristics.
The polymer gel constituting the matrix of the polymer electrolyte of the invention is preferably a polymer having an oxyethylene chain in which polymer chains derived from at least one monomer having a copolymerizable vinyl group are cross-linked with a cross-linking compound having two acryloyl groups and a (poly)oxyethylene group. The organic solvent supported by the polymer gel is prevented from leaking by controlling the cross-linking density and the length of the oxyethylene chain. The polymer gel of the invention exhibits sufficient mechanical strength for use as an electrolyte matrix.
Combined with a substance capable of intercalating and deintercalating lithium ions or metallic lithium as a negative electrode., the polymer electrolyte of the invention provides a high-performance lithium secondary battery which does not generate lithium dendrites or does not allow generated lithium dendrites to grow even during high rate charging. The polymer electrolyte of the invention holds a moderate amount of an electrolyte solution so that the hindrance of the polymer matrix to lithium ions migration is minimized. Because polymer chains cover the surface of the negative electrode to make the precipitation of lithium ions on the negative electrode surface uniform, lithium dendrites are hardly formed. If nuclei of dendrites generate, they are inhibited from growing by polymer networks.